Generally, a wheel may be rotated by the eccentric weight applied by the load applied to an axle mounted at an eccentric position of the wheel spaced horizontally from the center of the wheel rolling on the flat ground by a given distance, but if the axle mounted at the eccentric position of the wheel reaches the lower side of the wheel on a vertical line from the center of the wheel, the wheel is not kept rotated anymore by means of the load applied to the axle.